1) list givens and convert if necessary
C₁ = 3.50 mol/L
V₁ = 0.550 L
C₂ = ?
V₂ = 0.275 L
2) rearrange formula

3) solve

C₂ = 7.00 mol/L
I'm not exactly sure which one but I do know that an acid and a base react in a aqueous solution to form water, so i would probably eliminate the ones that aren't aqueous solutions.